The present invention relates to a dielectric filter module which is stable both mechanically and electrically, and particularly to a dielectric filter module employing a metalized dielectric block mounted within an open housing which supports adjustment screws for adjusting the characteristics of the filter.
The frequencies which are used in mobile communication systems has been raised from the VHF band to the UHF band and then to the microwave band, in order to meet the demand for a greater number of channels. The equipment used in such mobile radio communication systems is preferably small, lightweight, and economical. To help satisfy these requirements, the aforesaid related application proposes several embodiments of a small and rugged dielectric filter.
FIG. 1 illustrates an embodiment of the dielectric filter 10 of the aforesaid related application. Filter 10 has the trans-mission characteristics of a six-section Tchbyscheff filter having a resonance frequency of 876 MHz. Dielectric block 12 is generally brick-shaped, being about 80 millemeters long, 13.5 millemeters high, and 13.5 millemeters thick, and is made, for example, from Ti-Ba ceramic. The dielectric constant of block 12 is large. Six cylindrical resonator cavities 14 extend through block 12 from the top surface 16 to the bottom surface 18 thereof. A conductive film 20 is deposited on the inside surfaces of cavities 14 and extends to the outer surface of block 12, covering bottom surface 18, side surface 22, and opposite side surface 24. A suitable conductive film can be obtained by depositing an Ag-Pt film conductor about 15 mm thick to metalize the interior and exterior surfaces indicated. Top 16 and ends 26 of block 12 are not metalized. Metalizing the interior and exterior surface of block 12 in this way provides six quarter-wave resonators in which top surface 16 is an open plane and bottom surface 18 is a short-circuit plane. Since the dielectric constant of block 12 is large, the resonant electromagnetic field energy is mostly confined within block 12, which acts as a dielectric filter, and the external metalization provides a degree of electromagnetic shielding on three sides. Five cylindrical coupling adjustment cavities 27 are provided in block 12 between resonator cavities 14, the cavities 27 not being metalized.
With continuing reference to FIG. 1, block 12 is electrically and mechanically affixed within metal case 28 as by soldering. The ends of metal case 28 have holes 30 for receiving input/output connectors 32, which are secured to case 28 by screws 34. Input/output conductors 32 are electrically connected to block 12 via wires 36, which are connected to metal rods 38 inserted into cylindrical cavities 40 adjacent the ends 26 of block 12.
With continuing reference to FIG. 1, top surface 16 of block 12 is an open plane on which no metalized film has been deposited, so a very small quantity of electromagnetic field energy is emitted. Case 28 is therefore sealed by metal lid 42, which is secured to case 28 by screws 44. Six frequency adjustment screws 46 extend through lid 42, each screw 46 being positioned with respect to its corresponding resonator cavity 14 to permit fine control of the resonance frequency by effectively varying the length of the cavities. Similarly, five coupling adjustment screws 48 extend through lid 42 and are positioned coaxially with respect to their corresponding coupling adjustment cavities 27, thereby permitting fine adjustment of the coupling between the resonators.
As is explained in the aforesaid related application, which is incorporated herein by reference, the filter of FIG. 1 can be employed as a filter or antenna duplexer for separating transmitting and receiving waves for mobile radio equipment, so that an antenna may be used common. Filter 10 can be modified in a number of ways; for example, resonator cavities 14 need not extend entirely through block 12 and may be metalized for only a portion of their lengths, and the configurations and positions of coupling adjustment cavities 27 can be varied. Adjustment screws of metal or insulating material may be used. Moreover, in many applications a six resonator filter may not be needed, so that the number of cavities 14 and 27 and their corresponding screws 46 and 48 can be reduced. Such a simplification in structure is desirable for mobile communication systems, when electrical parameters permit, but due to financial considerations the weight and expense of metal case 28, lid 42, and screws 44 generally must remain unchanged in practice in spite of the decreased physical dimensions of the dielectric filter itself. That is, manufacturing economics may mandate that case 28 be available in only a limited number of sizes even if this means that the filter modules have cases that are larger than necessary. This is an obstacle to the mass production of light, inexpensive communication equipment.